Following a strong bleach of human rod or cone pigments it is known that the return of visual sensitivity directly reflects the return of the pigments. Following small bleaches such as occur in everyday life, however, the threshold changes to a disproportionately greater extent. This circumstance has been viewed by some as the intrusion of a separate mechanism of adaptation; by others, as a change in the gain of the basic mechanism by which pigments control sensitivity. It is known that the size of the test stimulus controls the slope of the delta I vs. I curve and, therefore, the extent of the dark adaptation curve. The proposed research will use a retinal densitometer that has been specifically constructed to allow measurements of both visual thresholds and retinal pigment densities with the same apparatus and with identical visual parameters. The effect of test stimulus size on sensitivity will be compared to pigment measurements with identical test areas during light adaptation, dark adaptation, and at such steady-state levels as are feasible. Attempts will also be made to compare the visual pigment changes during bleaching with sensitivity changes during light adaptation using identical visual parameters. This will require ancillary threshold equipment because of the high thresholds during light adaptation.